Apparatus for vaporizing monomers that flow at room temperature

ABSTRACT

An evaporator chamber has an entry end with a conduit connected to the output of a mass flow regulator, an exit end connected to a vacuum, and a wall surface extending therebetween. A body having capillary action is disposed in the chamber and has one end connected to the conduit and a free surface spaced from the wall surface in line of sight thereof. Heating means for the wall surface heats the body radiantly and causes monomer carried thereby to evaporate.

BACKGROUND OF THE INVENTION

The invention relates to apparatus for the evaporation of monomers whichare liquid at room temperature and have a low vapor pressure, especiallythose from the group TEOS (tetra-ethyl orthosilicate), TMS(tetramethylsilane) and HMDS (hexamethyldisiloxane), in the productionof thin layers containing silicon and oxygen by chemical vapordeposition (CVD) in a vacuum on substrates, through the use of a flowadjuster having an evaporator connected to its output.

Plasma processes and vapor deposition processes are used at pressuresbelow atmospheric pressure (vacuum) for producing coatings containingsilicon and oxygen. The starting monomers are liquids, such as TEOS, TMSand HMDS, which are characterized by a relatively low vapor pressure atroom temperature. On account of the extremely high purity required ofthese compounds, a so-called delayed boil often occurs when they areevaporated, i.e., an abrupt, explosive evaporation of relatively greatamounts of liquid, and hence an irregular mass flow. This property isespecially pronounced in the case of TEOS.

Through the article by B. L. Chin and E. P. van de Ven entitled, "PlasmaTEOS Process for Interlayer Dielectric Applications," published in"Solid State Technology," Apr. 1988, pages 119 to 122, it is known topump TEOS in liquid form into the vacuum reactor where it is evaporatedand mixed with auxiliary gases. In spite of a computer-controlled liquidpump with a repeatable throughput it is not possible to produce asufficiently constant mass flow on account of the intermittentevaporation of the liquid in the vacuum.

In considering these things it must be borne in mind that anyirregularity in he mass flow is reflected in irregular coatingproperties and, if it is a reactive process that is involved, there willalso be differences in the stoichometry of the coating composition.

Through the article by S. P. Mukherjee and P. E. Evans, "The depositionof thin films by the decomposition of tetra-ethoxysilane in aradio-frequency glow discharge," published in "Thin Solid Films," 1972,pp. 105-118, it is known to evaporate TEOS with a temperature-controlledevaporator which permits a defined increase of vapor pressure. In thiscase, too, continuous evaporation is virtually unachievable. If othermonomers are to be evaporated, the apparatus must be carefully adaptedto the vapor pressure curves of these monomers. Furthermore, the methodin question is limited in its application, since it is not possible toachieve an arbitrarily high vapor pressure, and the easily condensablevapors can be transported and metered only in an unsatisfactory manner.

U. Mackens and U. Merkt describe a very similar method with comparabledisadvantages in their article, "PLASMA-ENHANCED CHEMICALLYVAPOUR-DEPOSITED SILICON DIOXIDE FOR METAL/OXIDE SEMICONDUCTOR STRUCTUREON InSb," published in "Thin Solid Films," 1982, pages 53-61.

Through the article by R. E. Howard, "Selecting Semiconductor Dopantsfor Precise Process Control, Product Quality and Yield and Safety,"published in "Microelectronic Manufacturing and Testing," December 1985,pages 20-24, it is furthermore known to pass a noble gas (argon, helium)through a liquid monomer contained in a quartz vessel, in which thenoble gas entrains the monomers. Disadvantageous in this method is thelack of information on the amounts of the monomer actually reaching thereaction chambers. Furthermore, additional gases must always be put in,and in many applications the result is an unacceptable limitation inregard to the choice of the working parameters of pressure and gas flow.

Lastly, a method of the kind described above is disclosed in EP-OS 0 239664, in which a needle valve is used as the flow adjuster and isconnected to an evaporator not further described. The result of such anarrangement is that the monomer evaporates more or less uncontrolledlybehind the needle valve which forms a constriction. The only purpose theevaporator then serves is to evaporate any entrained liquid droplets andthus to assure complete evaporation. Consequently a constant mass flowcannot be achieved even with this known method.

The purpose of the invention is therefore to devise a method of the kinddescribed above, in which a constant, precisely controllable vapor flowor rate of flow can be sustained over a long period of time.

SUMMARY OF THE INVENTION

A mass flow regulator is used as the flow adjuster and the monomerpumped thereby is fed in the liquid state to the evaporator in which thecomplete evaporation is produced at the pressure of the vacuum by aheated body having a capillary action.

In a method of this kind the evaporation takes place not at anuncontrollable point behind the flow adjuster, but exclusively as wellas completely within the evaporator. The mass flow regulator itselfprovides for a liquid monomer feed that can be controlled withinextremely narrow limits. In other words, what is achieved is anextraordinarily controlled and precise feed of liquid monomers on theone hand, and on the other hand a very accurate evaporation at alocation provided and arranged for it, namely within the evaporator andwithin the heated body having capillary action.

The mass flow regulator used in this method is a precision apparatus inwhich the required throughput, i.e., the flow per unit time, is preset,and the actual throughput is controlled to within very slightdifferences from the preset throughput.

A mass flow regulator outstandingly appropriate for this purpose isoffered by Bronkkorst High-Tech B.V. of Ruurlo, Holland, under the modelnumber F 902-FA. The principle of the measurement of the actual massflow is based on a laminar flow in a heated conduit with the input of aconstant electrical power. By measuring the difference in temperaturebetween the input and the output of the conduit it is possible directlyto determine the magnitude of the mass flow. The measured difference iscompared with a preset value, and the result serves for the actuation ofa magnetic valve consisting of a nozzle and a target plate situated infront of it, whose distance from the nozzle is controlled by a solenoid.In this manner the mass flow can be regulated with extreme accuracy andalso controlled. The mass flow will be between 0.1 and 30 g/h at alinear characteristic with a maximum deviation of ± 0.83%.

It is especially advantageous to provide evaporation heat to the bodyhaving capillary action at least mostly by radiant heating.

In this manner the liquid monomer in any case does not contact theheated walls of the evaporator, so that no chemical changes of themonomer occur.

It is furthermore advantageous to give the body having the capillaryaction an elongated configuration and suspend it by its extremities, andto feed the liquid monomer to the one end of the body and withdraw theevaporated monomer from the free surface of the upper body and feed itinto the reaction zone in a vacuum chamber.

It will also be advantageous to produce in the body having the capillaryaction a temperature gradient of positive sign from the end at which theliquid monomer enters to the end at which the vaporized monomer emerges.

The invention furthermore relates to an apparatus for the practice ofthe method described above, through the use of a flow adjuster having anevaporator connected to its output. The evaporator has an evaporatingchamber with an entry end and an exit end, and a heating means forheating, and it is followed by a vacuum chamber for the performance of achemical vapor deposition process.

For the achievement of substantially the same purpose, such an apparatusis characterized in accordance with the invention in that the flowadjuster is a mass flow regulator and that in the evaporating chamber abody having a capillary action is disposed whose one end is connected tothe conduit coming from the mass flow regulator and which is held at adistance from the heated walls of the evaporator chamber but in a lineof sight with the latter.

Especially important is the body having the capillary action. It can bea porous ceramic and/or mineral material, a fleece, a fabric or a stringof a fibrous inorganic or organic material. The pore size or thedistance between the individual fibers determines the extent of thecapillary action. Such bodies can also be described as "sponge-like."

It is quite especially desirable to have the entry end of the bodyhaving the capillary action inserted into the conduit carrying theliquid monomer, and to have the longitudinal axis of the body inclineddownwardly at a low angle from the said chamber axis. In this manner theliquid monomer will enter the absorbent material at one end and migrate,while portions of it are continously evaporating, toward the exit end ofthe body having the capillary action. The design will at the same timebe such that no liquid will reach the exit end, so that all of theliquid monomer will be converted from the liquid to the vapor phasewithin the said body.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a complete installation for performinga CVD process,

FIG. 2 is an axial section through an evaporator in accordance with theinvention,

FIG. 3 is an enlarged detail from FIG. 2,

FIG. 4 is a diagram showing two curves indicating the pressure in theevaporator and in the receptacle from start-up to steady operation,

FIG. 5 is an Auger spectrogram of a coating which was made by the methodin accordance with the invention, in an apparatus in accordance withFIGS. 1 to 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a vacuum chamber 1 in which a CVD process is performed andwhich is connected to two vacuum pumps 2 and 3. In the vacuum chamber 1there is a substrate holder 4 which is connected by a conductor 5 to ahigh-frequency generator 6. The vacuum chamber 1 is at ground potential,and the conductor 5 is brought through the chamber wall by means of alead-through insulator 5a. On the substrate holder 4 are a plurality ofsubstrates 7 which are to be coated by the CVD process.

The liquid monomer is in a supply tank 8 and it is fed through a conduit9 to the mass flow regulator 10. From the latter in turn runs aliquid-carrying conduit 11 to an evaporator 12 which will be furtherexplained below in connection with FIGS. 2 and 3. From the evaporator 12a vapor line 13 runs through a shut-off valve 14 and another conduit 15to the vacuum chamber 1. An inert gas with which a glow discharge can besustained in the vacuum chamber 1 in the area of the substrate holder 4is fed into conduit 15 through a conduit 16 and a control valve 17,while additional doping and additive gases can be fed through anadditional conduit 18 and an adjusting valve 19 into the vacuumchamber 1. An air dryer 20 is connected to the top of the supply tank 8.

The entire system is centrally controlled by a control unit 21 which isconnected to the individual units by numerous conductors and conduitsrepresented by broken lines. Since the system according to FIG. 1belongs to the state of the art, with the exception of the mass flowregulator 10 and the evaporator 12, there is no need to go any furtherinto the manner of operation.

The evaporator 12 in FIG. 2 has an evaporating chamber 22 whoseessential part is formed by a hollow cylinder 23 which is surrounded bya heating coil 24. This heating coil consists of a central heatingconductor 25 and an insulating cover 26 surrounding it; the terminalends are not shown.

The hollow cylinder 23 is provided at both ends with annular flanges 27and 28. An end cap 31 is placed on the annular flange 27 at the entryend, with the interposition of an 0-ring 29 and a centering ring 30. Theend cap 31 has a threaded nipple 32 to which the liquid-carrying conduit11 (FIG. 1) is connected. Inside of the end cap 31 is a conduit 33 forcarrying the liquid monomer.

The vapor line 13 (FIG. 1) is connected to the flange 28 by an O-ring 34and a centering ring 35. The end cap 31 and the vapor line 13 haveflanges 36 and 37 complementary to the flanges 27 and 28. The outersides of the pairs of flanges 27, 36, and 28, 37, are tapered so thatthe flanges can be clamped against one another by pairs of hollowsemicircular clamping rings 38 and 39.

The heating coil 24 between the clamping rings 38 and 39 is surroundedby an outer jacket 40. The space between the outer jacket and the hollowcylinder 23 is filled with a thermal insulation material 41.

In the evaporating chamber 22 there is an elongated body 42 thatprovides a capillary action, one end 42a of which is connected to theconduit 33 coming from the mass flow regulator, i.e., one end [of thiselongated body] is inserted into this conduit. The evaporating chamber22 has a heated wall surface 22a, and it can be seen that the body 42 isdirectly exposed to this wall surface, without touching it. Due to thelow-angle of inclination of the body 42, the liquid monomer introducedinto the conduit 33 can more easily flow downhill through the body 42,while the evaporated monomer issues in the direction of the arrows fromthe entire surface of this body.

It can be seen in FIG. 2 that the heat of evaporation is supplied to thebody 42 having the capillary action almost exclusively by radiant heatfrom the wall surface 22a.

It can also be seen in FIG. 2 that a heated filter 45 is disposed on theexit end 44 of the evaporating chamber 22. This filter serves to preventany surface particles that come loose from the body 42 from entering thevacuum chamber 1. The heating serves to prevent monomer vapors fromcondensing in the filter itself.

As FIG. 3 shows, the body 42 having the capillary action consists of anelongated rod 46 on which the string of wick material 47 is woundhelically. Instead of this string of wick material, a tubular orsock-like wick can be used, which is drawn onto the rod 46.

In the case of the subject matter of FIG. 2, the liquid concentrationdecreases from left to right and the vapor concentration increases fromleft to right, while the temperature increases from left to right.

It has been observed that the precisely metered liquid evaporatessmoothly and controlledly and issues at the surface 43.

In FIG. 4 the abscissa is the time axis and the ordinate the pressure.The pumping down of the vacuum chamber, and hence the evacuation of theentire system, begins at time T₀. At time T₁ the operation of theevaporator 12 begins, and at time T₂ the pressure curves become steady.The bottom curve 48 indicates the pressure in vacuum chamber 1, whilethe upper curve 49 shows the pressure in the evaporator 12. It can beseen that these pressures follow a very constant and rectilinear course.

In FIG. 5, the left margin of the diagram represents the surface of thecoating on the substrate. Starting at this surface an Auger electron ionbeam analysis was performed. The very straight and level shape of theconcentration curves for silicon and oxygen up to about 0.5 show theprecision of the coating that was formed. This is to be attributed tothe very precise regulation of the amount of monomer evaporated.

The inventive apparatus is useful in a preferred method of producingsilicon dioxide coatings from TEOS for microelectronic applications.

The vacuum chamber shown in FIG. 1 is an apparatus of the model Z 550made by Leybold AG, in which the substrate holder 4 is operated at afrequency of 13.65 MHz. The substrates used were silicon wafers with adiameter of 100 mm, on which vertical aluminum test steps were disposedwith a rise of 750 mm and a distance of about 2 microns between steps.The CVD process was performed, after an appropriate previous treatmentof the substrates, at a working pressure of 2×10⁻² mbar, the TEOS beingfed at a rate of 10 g/h. At the same time oxygen was admixed at 100sccm. Evaporator 12 was kept at a temperature of 150° C.; the substratetemperature was about 100° C. The coating time was 15 minutes.

We claim:
 1. Apparatus for the evaporation of monomers which are liquidat room temperature and have a low vapor pressure in the production ofthin coatings containing silicon and oxygen by chemical vapor depositionin a vacuum on substrates, said apparatus comprising:a mass flowregulator for adjusting the flow of the monomer, an evaporatorcomprising an evaporating chamber having an entry end, an exit end, anda wall surface extending therebetween, said entry end comprising aconduit connected to the output of said mass flow regulator, said exitand being connected to said vacuum, said evaporator further comprisingmeans for heating said chamber, and a body having capillary actiondisposed in said chamber, said body having one end connected to theconduit, said body having a free surface spaced at a distance from thewall surface and in line of sight of the wall surface.
 2. Apparatus inaccordance with claim 1 characterized in that the evaporating chamber isconstructed as a hollow cylinder surrounded by the heating means andhaving a wall at the entry end, into which the conduit carrying theliquid monomer leads, and that the body having the capillary action isdisposed in the hollow cylinder in such a manner that its one end isfacing the conduit carrying the monomer, and that its free surface facesthe wall surface of the evaporating chamber.
 3. Apparatus in accordancewith claim 2, characterized in that said one end of the body having thecapillary action is inserted into the conduit carrying the liquidmonomer.
 4. Apparatus in accordance with claim 1, characterized in thatthe body having the capillary action comprises a wick material. 5.Apparatus in accordance with claim 4, characterized in that the bodyhaving the capillary action further comprises an elongated supportingmember which is surrounded by said wick material.
 6. Apparatus inaccordance with claim 5, characterized in that the wick material iswound on the supporting member.
 7. Apparatus in accordance with claim 1,further comprising a heated filter disposed on the exit end of theevaporating chamber.
 8. Apparatus as in claim 1 wherein said means forheating said chamber comprises means for heating said wall surface,resulting in a heated wall surface which heats said body havingcapillary action by radiant heat.
 9. Apparatus as in claim 1 whereinsaid body has a longitudinal axis which runs downward from said entryend toward said exit end.